HTLV-I and HTLV-II have similar genome organization and similar protein profiles sharing 65% nucleic acid homology and 70% amino acid homology. We generated murine monoclonal antibodies to HTLV-II viral proteins that react with core (gag) and envelope (env) proteins. BALB/c mice were immunized with purified HTLV-II virus. ELISA and Western blots were used to screen for and to confirm positive clones. Monoclonals that react with the core proteins p24 were identified. These monoclonals show preferential reactivity to HTLV-II p24 but cross-react with the HTLV-I p24 protein. We also isolated a monoclonal antibody, M5H6, which reacts specifically with the gp46 envelope protein of HTLV-II. Western blot analysis using this monoclonal showed reactivity to a diffuse glycoprotein of apparent Mr. of 46,000 which was detected only in HTLV-II infected cell or purified viral preparations. The M5H6 monoclonal reacted with a recombinant protein derived from HTLV-II gp46 (aa 162-205) mapping the relevant epitope to this region. M5H6 binds to HTLV-II infected cells in immunofluorescent assays. Other studies characterizing these monoclonals will be presented.